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<a name="Improved-copy"></a>
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Next: <a href="Improved-m4wrap.html#Improved-m4wrap" accesskey="n" rel="next">Improved m4wrap</a>, Previous: <a href="Improved-foreach.html#Improved-foreach" accesskey="p" rel="prev">Improved foreach</a>, Up: <a href="Answers.html#Answers" accesskey="u" rel="up">Answers</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Solution-for-copy"></a>
<h3 class="section">17.4 Solution for <code>copy</code></h3>

<p>The macro <code>copy</code> presented above
is unable to handle builtin tokens with M4 1.4.x, because it tries to
pass the builtin token through the macro <code>curry</code>, where it is
silently flattened to an empty string (see <a href="Composition.html#Composition">Composition</a>).  Rather
than using the problematic <code>curry</code> to work around the limitation
that <code>stack_foreach</code> expects to invoke a macro that takes exactly
one argument, we can write a new macro that lets us form the exact
two-argument <code>pushdef</code> call sequence needed, so that we are no
longer passing a builtin token through a text macro.
</p>
<dl>
<dt><a name="index-stack_005fforeach_005fsep"></a>Composite: <strong>stack_foreach_sep</strong> <em>(<var>macro</var>, <var>pre</var>, <var>post</var>,   <var>sep</var>)</em></dt>
<dt><a name="index-stack_005fforeach_005fsep_005flifo"></a>Composite: <strong>stack_foreach_sep_lifo</strong> <em>(<var>macro</var>, <var>pre</var>,   <var>post</var>, <var>sep</var>)</em></dt>
<dd><p>For each of the <code>pushdef</code> definitions associated with <var>macro</var>,
expand the sequence &lsquo;<samp><var>pre</var>`'definition`'<var>post</var></samp>&rsquo;.
Additionally, expand <var>sep</var> between definitions.
<code>stack_foreach_sep</code> visits the oldest definition first, while
<code>stack_foreach_sep_lifo</code> visits the current definition first.  The
expansion may dereference <var>macro</var>, but should not modify it.  There
are a few special macros, such as <code>defn</code>, which cannot be used as
the <var>macro</var> parameter.
</p></dd></dl>

<p>Note that <code>stack_foreach(`<var>macro</var>', `<var>action</var>')</code> is
equivalent to <code>stack_foreach_sep(`<var>macro</var>', `<var>action</var>(',
`)')</code>.  By supplying explicit parentheses, split among the <var>pre</var> and
<var>post</var> arguments to <code>stack_foreach_sep</code>, it is now possible to
construct macro calls with more than one argument, without passing
builtin tokens through a macro call.  It is likewise possible to
directly reference the stack definitions without a macro call, by
leaving <var>pre</var> and <var>post</var> empty.  Thus, in addition to fixing
<code>copy</code> on builtin tokens, it also executes with fewer macro
invocations.
</p>
<p>The new macro also adds a separator that is only output after the first
iteration of the helper <code>_stack_reverse_sep</code>, implemented by
prepending the original <var>sep</var> to <var>pre</var> and omitting a <var>sep</var>
argument in subsequent iterations.  Note that the empty string that
separates <var>sep</var> from <var>pre</var> is provided as part of the fourth
argument when originally calling <code>_stack_reverse_sep</code>, and not by
writing <code>$4`'$3</code> as the third argument in the recursive call; while
the other approach would give the same output, it does so at the expense
of increasing the argument size on each iteration of
<code>_stack_reverse_sep</code>, which results in quadratic instead of linear
execution time.  The improved stack walking macros are available in
<samp>m4-1.4.18/examples/stack_sep.m4</samp>:
</p>
<div class="example">
<pre class="example">$ <kbd>m4 -I examples</kbd>
include(`stack_sep.m4')
&rArr;
define(`copy', `ifdef(`$2', `errprint(`$2 already defined
')m4exit(`1')',
   `stack_foreach_sep(`$1', `pushdef(`$2',', `)')')')dnl
pushdef(`a', `1')pushdef(`a', defn(`divnum'))
&rArr;
copy(`a', `b')
&rArr;
b
&rArr;0
popdef(`b')
&rArr;
b
&rArr;1
pushdef(`c', `1')pushdef(`c', `2')
&rArr;
stack_foreach_sep_lifo(`c', `', `', `, ')
&rArr;2, 1
undivert(`stack_sep.m4')dnl
&rArr;divert(`-1')
&rArr;# stack_foreach_sep(macro, pre, post, sep)
&rArr;# Invoke PRE`'defn`'POST with a single argument of each definition
&rArr;# from the definition stack of MACRO, starting with the oldest, and
&rArr;# separated by SEP between definitions.
&rArr;define(`stack_foreach_sep',
&rArr;`_stack_reverse_sep(`$1', `tmp-$1')'dnl
&rArr;`_stack_reverse_sep(`tmp-$1', `$1', `$2`'defn(`$1')$3', `$4`'')')
&rArr;# stack_foreach_sep_lifo(macro, pre, post, sep)
&rArr;# Like stack_foreach_sep, but starting with the newest definition.
&rArr;define(`stack_foreach_sep_lifo',
&rArr;`_stack_reverse_sep(`$1', `tmp-$1', `$2`'defn(`$1')$3', `$4`'')'dnl
&rArr;`_stack_reverse_sep(`tmp-$1', `$1')')
&rArr;define(`_stack_reverse_sep',
&rArr;`ifdef(`$1', `pushdef(`$2', defn(`$1'))$3`'popdef(`$1')$0(
&rArr;  `$1', `$2', `$4$3')')')
&rArr;divert`'dnl
</pre></div>


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